A color cathode ray tube comprises basically electron guns for emitting electron beams and a phosphor screen that emits light upon irradiation with the electron beams so as to form a visible image. The electron beams are deflected by the geomagnetism so as to generate a color drift in the visible image formed on the phosphor screen. For preventing the color drift, an inner magnetic shield, which is also called an inner shield, is arranged in the color cathode ray tube in general.
In recent years, commercial TV sets have been enlarged or widened in the screen size. As a result, the flight path length and the scanning length of the electron beams have been increased and, thus, the TV sets have become more susceptible to the effect of geomagnetism. In other words, a deviation of the landing point on the phosphor screen of the electron beam from the designated point, which is caused by the effect of geomagnetism (thus termed a geomagnetic drift), has become larger than before. At the same time, a finer screen has come to be handled because of the propagation of the hi-vision broadcasting and the initiation of the digital broadcasting, with the result that demands for reduction of the geomagnetic drift has becomes severer. On the other hand, since a finer still image is required in a color cathode ray tube for a personal computer, it is more necessary to suppress markedly the color drift caused by the geomagnetic drift.
Under the circumstances, it was customary in the past to evaluate in many cases the characteristics of the steel sheet used for preparation of the magnetic shields by using as indexes the magnetic permeability, the coercive force and the remanent flux density under a low magnetic field corresponding substantially to the geomagnetism.
A method of improving the characteristics of the steel sheet for magnetic shields is disclosed in, for example, Japanese Patent Disclosure (KOKAI) No. 10-168551. This prior art is directed to the technology for improving the magnetic characteristics by setting the ferrite crystal grain size of 3 to 20 μm of a steel having a specified composition. To be more specific, disclosed in this prior art are a magnetic shield material exhibiting a coercive force not smaller than 3 Oe and a remanent flux density not lower than 9 kG, which are the magnetic characteristics required for the cold rolled steel sheet used for preparation of magnetic shields, and a method of manufacturing the particular magnetic shield material.
In an article, Transaction (in Japanese) of the Institute of Electronics, Information, and Communication Engineers, vol. J79-C-II No. 6, pp. 311–319, June 1996, disclosed is the relationship between the anhysteretic magnetic permeability and the magnetic shielding effect required for improving the magnetic shielding effect.
It should be noted in this connection that the steel sheet for magnetic shields applied to the actual color cathode ray tube is demagnetized in general under the geomagnetism and, thus, the magnetic characteristics of the steel sheet are changed by the demagnetization under the geomagnetism. However, the particular change in the magnetic characteristics are not taken into account in the technology disclosed in Japanese Patent Disclosure No. 10-168551 pointed out above, leading to the problem that the magnetic shielding effect is insufficient.
The relationship between the anhysteretic magnetic permeability and the magnetic shielding effect is studied in the article, Transaction (in Japanese) of the Institute of Electronics, Information, and Communication Engineers, vol. J79-C-II No. 6, pp. 311–319, June 1996, pointed out above. However, the detailed studies as to what steel sheet exhibits a high anhysteretic magnetic permeability are not clarified in this article.
As pointed out above, the technology disclosed in each of the prior arts referred to above is incapable of sufficiently coping with the deterioration in the visible image formed on the phosphor screen, which is caused by the color drift accompanying the enlargement achieved in recent years in the phosphor screen of commercial TV sets. Also, the color drift problem has not yet been resolved in respect of the cathode ray tube for personal computers.
Under the circumstances, strongly required nowadays is a high performance steel sheet for magnetic shields having a magnetic shielding effect.